Satellite images enable quick nationwide estimates of storm damage in forests

Remember Burglind? On 2 and 3 January 2018, the depression brought us wind storms of up to 201 km/h, which felled or snapped thousands of trees in Switzerland. Forest experts were soon swarming all over the country to assess all the forest areas affected as the federal government requires roughly two weeks after a big windthrow event estimates of the quantity of wood felled by the storm. Each canton then reported the total volume of wood to the Federal Office for the Environment (FOEN). In the whole country about 1.3 million m3 of wood lay on the ground, i.e. approximately a quarter of the average amount of wood harvested per year – only Ticino was spared.

Researchers at WSL wondered whether analysing satellite images would not be a quicker and cheaper way of gaining a first impression of the damage. They therefore obtained, together with specialists from the Remote Sensing Laboratories at the University of Zurich, images from the satellite-pair Sentinel-1, which scan the Earth’s surface with radar sensors. They then compared, with software help, the images the satellites took before and after the storm. The initial results were dis­appointing. “In some areas the windthrow was depicted realistically; in others, however, not. This probably has mainly to do with Switzerland’s complex topography,” says Marius Rüetschi from the Remote Sensing Group at WSL. Snow posed a further problem because the software often failed to register wood lying on the ground covered with snow as storm damage.

The experts have, nevertheless, managed to deal with the methodological and technical uncertainties. In collaboration with several cantons, they inspected some storm-damaged areas in the field and compared their observations with the analyses of the satellite images. The result: while the computer programme identified large areas of forest disturbed by the storm rather accurately, it registered smaller groups of felled trees less well.
“This technology opens up a promising path,” says Marius, “even though more factors influence the accuracy of such analyses than we first thought.” He therefore views the future with confidence: “All we need now is another storm,” he declares with a twinkle in his eye. (Reinhard Lässig, Diagonal 2/18)